The present invention relates to a spindle device which rotates a spindle holding a tool such as a whetstone at a high speed, and more particularly to a spindle device which converts a fluid energy of gas or liquid into a rotating power of a spindle using a turbine rotor.
Recently as a spindle device used for a machine tool such as a grinding machine, to meet the demand for a high precision machining to work pieces, a device which is small-sized and yet has the high rotational speed of a spindle has been required. The applicant of this application has proposed a spindle device which rotatably drives a spindle using a turbine rotor and supports the rotation of the spindle by means of dynamic pressure bearings (Japanese Patent Laid-open No.175137/1998).
In such a spindle device, while the turbine rotor having turbine blades is fixedly secured to the spindle, the rotation of the spindle is supported by the dynamic pressure bearings which generate a high-pressure fluid lubrication film. When a drive fluid is blown off to the turbine blade, the spindle is rotated together with the turbine rotor. Further, since the rotation of the spindle is supported by the dynamic pressure bearings, once the rotation of the spindle is started, the spindle is held in a floating state due to the action of the fluid lubrication film so that spindle is smoothly rotated substantially without being subjected to a resistance or a vibration.
Further, in this spindle device, to facilitate the recovery of the drive fluid and to miniaturize the turbine rotor, the turbine blades mounted on the turbine rotor are constituted as centrifugal blades. As the drive fluid passes through the inside of the turbine rotor in the radial direction, a rotation is given to the turbine rotor. Accordingly, the drive fluid is blown to the center of the rotating turbine rotor from a blow-off nozzle which is fixedly secured to a housing side and when the drive fluid passes through the inside of the turbine rotor radially, a rotational drive force is generated by the turbine blades.
However, in such a conventional spindle device, when the drive fluid is blown into the center of the turbine rotor with the use of the blow-off nozzle, the drive fluid is leaked into a gap formed between the housing to which the blow-off nozzle is fixedly secured and the rotating turbine rotor so that it has been difficult to blow a whole quantity of the drive fluid supplied to the blow-off nozzle into the turbine rotor. Further, since the leaked drive fluid generates a static pressure between the housing and the turbine rotor, a force is liable to act on the spindle in the direction to enlarge the gap between the housing and the turbine rotor and hence, there has been a tendency that a leaked quantity of the drive fluid is steadily increased. Accordingly, even when a supply pressure of the drive fluid to the blow-off nozzle is increased, the increase of the rotational torque of the spindle is extremely small so that there has been a problem that it is difficult to achieve a remarkable enhancement of the efficiency of machining of work pieces. As a method for preventing such a leakage of the drive fluid, a method which provides a contact seal such as a mechanical seal between the housing and the turbine rotor is named. However, since the turbine rotor is rotated at an extremely high speed, there have been disadvantages that it gives rise to a problem with respect to the durability of the seal and the structure becomes complicated.
The present invention has been made in view of these problems and it is an object of the present invention to provide a spindle device which can prevent the leakage of the drive fluid for a turbine rotor into a gap between a fixedly-secured housing and a spindle which is rotatably supported relative to the housing as much as possible whereby the supplied fluid liquid is efficiently utilized thus enhancing the rotational torque of the spindle.
To achieve the above mentioned object, according to the spindle device of the present invention, in a spindle device constituted such that the spindle includes a housing, a spindle which is rotatably supported on the housing and a turbine rotor which is fixedly mounted on the spindle so as to give a rotation to the spindle, and a drive fluid is blown off to the turbine rotor such that the drive fluid passes through a drive fluid passage formed along an axis of the spindle and is radially blown off toward a radial outside of the turbine rotor, a rotary sleeve which constitutes a radial dynamic pressure bearing is arranged on the spindle, a fixed sleeve of the radial dynamic pressure bearing which faces the rotary sleeve in an opposed manner by way of a given bearing gap is provided to the housing, the rotation of the spindle to the housing is supported by the radial dynamic pressure bearing, supply holes which supply the drive fluid to the drive fluid passage of the spindle from a housing side are radially formed in the fixed sleeve, and receiving holes which receive the drive fluid from the supply holes and introduce the drive fluid into the drive fluid passage are radially formed in the rotary sleeve.
In the spindle device of the present invention having such a constitution, although the spindle is rotatably driven by the turbine rotor, the drive fluid which drives the turbine rotor is first blown into the drive fluid passage of the spindle from the housing side and thereafter is blown to the turbine rotor fixedly mounted on the spindle. The drive fluid passage is formed along the axis of the spindle and the drive fluid is radially blown to the turbine rotor from the drive fluid passage to the radially outside of the turbine rotor in the radial direction. Here, the transfer of the drive fluid from the housing to the spindle which is rotated at a high speed is performed in the inside of the radial dynamic pressure bearing which supports the rotation of the spindle. That is, while the radial supply holes are formed in the fixed sleeve which constitutes the radial dynamic pressure bearing and is mounted on the housing, the radial receiving holes which receive the drive fluid from the supply holes and introduce the drive fluid into the drive fluid passage are formed in the rotary sleeve which also constitutes the radial dynamic pressure bearing and is mounted on the spindle, and the transfer of the drive fluid is performed between the supply holes and the receiving holes.
Here, the bearing gap formed between the fixed sleeve and the rotary sleeve which constitute the radial dynamic pressure bearing is several xcexcm and hence is extremely small. Further, during the rotation of the spindle, a fluid lubrication film of a high pressure is formed in the bearing gap. Accordingly, it is possible to prevent the drive fluid blown off from the supply holes from being leaked into the gap formed between the fixed sleeve and the rotary sleeve, that is, the bearing gap of the radial dynamic pressure bearing as much as possible. Accordingly, it becomes possible to make an approximately whole quantity of the drive fluid blown off from the supply holes flow into the receiving holes of the rotary sleeve. In other words, the radial dynamic pressure bearing performs a function of a seal to prevent the leakage of the drive fluid. Accordingly, at the time of blowing the drive fluid for the turbine rotor into the drive fluid passage of the spindle from the housing, the leakage of the drive fluid can be prevented as much as possible so that the rotational torque generated by the turbine rotor is increased by an amount corresponding to the leakage prevented drive fluid.
In such a technical means, the bearing gap of the radial dynamic pressure bearing is extremely small and hence, a quantity of the drive fluid which flows into the bearing gap without flowing into the receiving holes of the rotary sleeve is extremely small. However, the drive fluid blown off from the supply holes is pressurized, when the pressurizing force is high, there is a possibility that the drive fluid flows into the gap between the fixed sleeve and the rotary sleeve. Accordingly, from this point of view, it is preferable to constitute the spindle device such that a pair of pressure generating grooves are formed in an outer peripheral surface of the rotary sleeve or an inner peripheral surface of the fixed sleeve in such a manner that the pressure generating grooves sandwich the receiving holes and the supply holes so as to generate fluid lubrication films of a high pressure in the bearing gaps of the radial dynamic pressure bearings at both sides of the supply holes and the receiving holes. Due to such a constitution, the fluid lubrication films of a high pressure are formed at both sides of the supply holes and the receiving holes so that it becomes possible to positively prevent the drive fluid blown off from the supply holes from flowing into the bearing gaps of the radial dynamic pressure bearings whereby it becomes possible to prevent the loss of the drive fluid transferred from the supply holes to the receiving holes as much as possible.
Further, since the spindle is rotated, depending on the rotational position of the spindle, there may be a case that the communication between the supply holes formed in the fixed sleeve and the receiving holes formed in the rotary sleeve is interrupted. When the supply of the drive fluid to the turbine rotor is interrupted due to such an interruption of communication, the rotational torque of the spindle is remarkably fluctuated. Accordingly, from this point of view, it is preferable to respectively form a plurality of supply holes or receiving holes in the circumferential direction of the fixed sleeve or the rotary sleeve such that any one of the supply holes and any one of the receiving holes are always communicated with each other irrespective of the rotational position of the spindle. Due to such a constitution, the supply of the drive fluid to the turbine rotor is not interrupted and hence, it becomes possible to prevent the fluctuation of the rotational torque given to the spindle as much as possible.
Further, in the spindle device of the present invention, the drive fluid for the turbine rotor is blown off to the spindle such that the drive fluid passes through the radial dynamic pressure bearing in the radial direction and hence, it may be possible to use the lubrication fluid supplied into the bearing gap of the radial dynamic pressure bearing in common with the drive fluid. However, in case the drive fluid and the lubrication fluid are used in common, when the supply of the drive fluid is stopped for stopping the rotation of the spindle, the supply of the lubrication fluid to the radial dynamic pressure bearing is also stopped. In this case, since the bearing gap of the radial dynamic pressure bearing receives the remarkable negative pressure, there is a possibility that the fixed sleeve and the rotary sleeve are adhered to each other thus giving rise to a solid contact. Accordingly, from this point of view, it is preferable to constitute the spindle device such that a supply passage of the lubrication fluid to the bearing gaps is formed independently or separately from a supply passage of the drive fluid for the turbine rotor such that even at the time of stopping the supply of the drive fluid, the lubrication fluid is supplied to the bearing gaps of the radial dynamic pressure bearing.
Still further, a liquid such as water, a coolant liquid or the like can be used as the drive fluid and the lubrication fluid in addition to a gas such as air or the like.